This invention relates to an apparatus for carrying out gas separation by way of Pressure Swing Adsorption (PSA) and Vacuum Swing Adsorption (VSA) processes, in particular those processes in which relatively small absolute pressure swings, at close to ambient pressures, are involved (ie Low Pressure Adsorption (LPA) processes).
Conventional adsorption apparatus comprises at least one vessel containing one or more beds of adsorbent material. Gases to be separated are passed vertically through the adsorbent beds, each of which is no more than 1 to 2 m in height, due to the maximum permissible pressure drop as the gas flows through the bed. Multiple beds are usually arranged in series within a vessel in order to maximise adsorption. The vessels containing the adsorbent beds are usually cylindrical, with convexly dished ends, since such a configuration makes for a robust pressure vessel having reliable and easily-calcuable performance characteristics when subjected to internal or external pressure.
These cylindrical pressure vessels are designed for use in either a vertical or a horizontal position, the adsorbent beds contained therein normally being designed to operate with vertical gas flows therethrough. Generally, vertical vessels are preferred since in practice these have a constant vertical cross-section and thus give the best, most uniform gas flow distribution. However, in order to optimise adsorption performance, as mentioned above, multiple beds are used, which leads to very tall pressure vessels, which cannot be transported except in the horizontal position. This can damage or adversely affect the adsorbent beds therein unless these have been specifically designed to undergo such treatment, which leads to unnecessary design and manufacturing expense.
To address the transport limitations associated with vertical vessels, some manufacturers have used horizontal pressure vessels, which are relatively easily transported whilst containing the adsorbent beds. However, the variable cross-sectional area of the adsorbent bed or beds (in the vertical direction of gas flow) leads to a variable gas flow distribution within the bed and this gives rise to process losses.
A further problem with large gas separation plants is due to the high temperature differences which are generated within the adsorbent beds. These can be very significant, and on some PSA plants have been measured at +60xc2x0 C. at the top of a 2 m high bed, and xe2x88x9290xc2x0 C. at the bottom of the bed. Needless to say, such a wide temperature variation along the effective height of the bed gives rise to poor performance; losses of up to 60% have been noted in some instances.
A temperature gradient across the gas flow discourages uniform gas flow distribution, thus preventing the creation of xe2x80x9cfrontsxe2x80x9d of gas as a flow passes through the bed and results in premature xe2x80x9cbreak throughxe2x80x9d at the top of the bed (this phenomenon is well known and documented in the art, and therefore not described in detail here). The consequent loss of performance can also approach 60% (measured on a plant having a xe2x88x9290xc2x0 C. cold spot at the vessel centre and a xe2x88x9210xc2x0 temperature at the vessel wall, across the direction of gas flow).
Accordingly, apparatus for effecting adsorptive separation of at least one gaseous component comprising at least one transportable cargo container having a pair of side walls, a pair of end walls, a base and a roof, wherein the pair of side walls, the base, the roof, and at least one of the end walls define a vessel for containing at least one bed of adsorbent material, and wherein the apparatus is adapted to operate on a low pressure swing not exceeding 3 bar absolute.
Such an apparatus is not only significantly simpler and cheaper than a conventional cylindrical vessel to manufacture but is far more easily transported, given the familiarity of the haulage industry with standard, transportable ISO freight containers, which the vessel of the present invention can be made to resemble (at least outwardly). Despite the lesser structural strength of such a configuration as compared to a cylindrical vessel, a rectilinear vessel can easily be made sufficiently robust to withstand the relatively low pressure differences relative to ambient pressure which are typical in LPA processes; accordingly the vessel of the present invention is particularly suited for these processes. In practice, the functioning of the vessel of the present invention is less affected (for example, by atmospheric pressure affecting the vessel""s structural integrity or allowing air ingress or gas egress) when under pressure than under vacuum, and accordingly is best suited at pressures between xe2x88x921 and +2 bar gauge. A further advantage of the above-described arrangement is that the vessel can easily be adapted to contain one or more adsorbent beds spaced horizontally and configured for vertical gas flows therethrough, and the substantially vertical walls of transportable containers give a constant cross-section transverse to the direction of gas flow.
The vessel may be provided with bracing means which are mounted to, extend between and are adapted structurally to brace the side walls of the container, the bracing means being adapted thermally to conduct heat into and from localised areas within the bed which areas, in use, become cold and hot in the direction, from and toward the side walls respectively.
In this way the means providing the structural support, which is typically necessary as absolute pressure swings increase towards 2 bar, can also function as means for xe2x80x9csmoothing outxe2x80x9d the temperature gradients created in the adsorbent bed in use. Aluminium, brass or aluminium alloy are suitably strong and thermally-conductive materials.
The bracing means preferably comprise a plurality of plates disposed substantially parallel to the end walls, so as to permit free movement of gas in the vertical direction, and to provide thermal conduction in both the vertical and one horizontal plane. These plates may be sized relative to and disposed within the vessel so as to permit at least limited movement of gas and/or adsorbent in directions generally perpendicular to the plates. This maintains a substantially level top surface to the bed of particulate adsorbent. To assist this function, the plates may be perforated. The plates may be formed of aluminium, aluminium alloy, brass or steel.
As mentioned above, the apparatus is preferably adapted for vertical gas flows. To achieve this, the or each container may be provided with means adapted to supply and distribute the mixed gas stream into the bed of adsorbent material at the base of the container. To cope with the gas flow reversals necessary in most normal gas separation processes, the supply and distribution means may be sized and configured for the collection and withdrawal of a mixed gas stream. Additionally or alternatively the apparatus may comprise means adapted to collect and withdraw gas passing from the bed of adsorbent material, and/or means to supply and distribute purging gas into the bed of adsorbent material, located adjacent the roof of the container.
A complete LPA system may be contained within a single container, or the system may comprise a plurality of containers which can be connected in modular fashion to produce a working system.
In the former case the apparatus for separating a gaseous component from, for example, air, may comprise a single vessel sealingly partitioned into two or three sections by means of one or two partition walls substantially parallel to the end walls, the sections adjacent the end walls being of substantially equal volume and each being adjusted to contain a bed of adsorbent material.
Alternatively, the roof of the or each container, or at least a major portion thereof, may be releaseably attached. With such an arrangement pairs of containers can be connected together by means of a gas tight seal extending around the periphery of the contiguous roofs, the releasable roofs or portions thereof having been detached.